Polyphenylene sulfide (PPS) is also known as thiophenylene, which has a symmetrical macromolecular linear rigid structure formed by association between phenyl ring and sulfur atom; its molecular structure is shown as follows:

Polyphenylene sulfide is an excellent special engineering plastic, which serves as the sixth universal engineering plastics following polycarbonate (PC), polyester (PET), polyformaldehyde (POM), polyamide (PA) and polyphenylene oxide (PPO) as well as one of the eight aerospace materials. Owing to its chemical structure featuring in alternating and ordered arrangement of sulfur atoms and phenyl rings, polyphenylene sulfide can ensure highly stable chemical bond of molecules, which has such features as high temperature and radiation resistance, flame retardancy, low viscosity, high dimensional stability, high resistance to solvents and chemical corrosion, excellent dielectrical properties and wear resistance. Its major physiochemical properties are stated as follows:
(1) High temperature resistance: PPS has excellent thermal performance with melting point and thermal deformation temperature over 280° C. and 260° C. respectively; it can withstand an instantaneous temperature of 260° C. and is available for long-term application below the temperature of 200° C. As its thermal deformation temperature is up to 350° C., it is presently one of the thermoplastic engineering plastics with maximum working temperature.
(2) Chemical resistance: High chemical resistance is one of the most distinctive features of PPS; its chemical stability is just next to polytetrafluoroethene (PTEE). PPS is stable to most acids, esters, ketones, aldehydes, phenols as well as fatty, aromatic, chlorinated hydrocarbons, etc. So far, no solvent that can dissolve polyphenylene sulfide below the temperature of 200° C. has been discovered yet. PPS has a high resistance to inorganic acid, alkali and salts. It is only soluble in diphenyl and diphenyl ethers as well as their halides above the temperature of 250° C.
(3) Excellent electrical properties: PPS has excellent electrical properties; its dielectric constant and tangent value of dielectric loss angle are relatively low as compared with other engineering plastics, which can remain almost unchanged in case of excessive frequency, temperature and temperature ranges. PPS also has high resistance to electric arc, which is on par with thermosetting plastics. PPS is frequently used as insulating material for electrical appliances, of which consumption accounts for approximately 30% of the total consumption amount of the PPS.
(4) Flame retardancy and wear resistance: PPS has an oxygen index of 46-53, which is available for combustion on the flame other than dripping. It is to be extinguished automatically once moved away from the flame, of which smoke production is lower than that of halogenated polymers. It is up to the high flame retardancy criteria of UL-94V-O without addition of any fire retardant. It is applicable to fill in fluororesin and carbon fiber lubricant to significantly improve wear resistance and properties of PPS.
(5) Perfect machining performance: It is applicable to produce PPS by means of injection, mold pressing and extrusion to ensure relatively low forming shrinkage, linear expansion coefficient and water absorption. Its products are free of any deformation in the environment of high temperature and humidity.
Owing to its excellent properties and perfect compatibility to inorganic fillers, reinforced fibers and other polymer materials, polyphenylene sulfide can be fabricated into various reinforced fillers and polymer alloys. It has extensive applications, which is mainly used in such industries as electronic and electrical appliances, precise instruments, machineries, vehicles, household appliances, films, fibers, power supply, aviation, environmental protection and chemistry.
As reported by literatures, there are numerous methods for synthesis of PPS. However, there is only one real industrialized method for synthesis of PPS through polycondensation between sulfides and polyhalogenated aromatic hydrocarbons. Sulfides are mainly represented by sodium sulfide and sodium bisulfide or hydrogen sulfide as reported by literatures. Working principle of hydrogen sulfide is almost identical to that of sodium sulfide. The only difference lies in the supplemented reaction for generation of sodium sulfide by using sodium hydroxide to absorb sodium bisulfide gas. Presently, most of PPS manufacturers and research institutions are concentrated in USA, Japan and China.
Phillips Petroleum Company developed a method for industrialized synthesis of PPS in 1960s and 1970s. The company proposed for the first time to use sodium sulfide and dichlorobenzene as materials and N-methyl-2-pyrrolidone as the solvent for production of PPS resin with melting point over 275° C. through dehydration and polycondensation in the U.S. Pat. No. 3,354,129, and realized industrialized production of PPS for the first time in 1971. Its products were later put into the market under the commodity name of “Ryton”. Owing to low price and easy acquisition of materials, limited technical route, stable production quality and high yield, the method for synthesis of PPS by using sodium sulfide as material has attracted high attentions. For similar synthesis methods, one can refer to U.S. Pat. No. 3,487,454, U.S. Pat. No. 5,393,865, U.S. Pat. No. 3,867,356, U.S. Pat. No. 4,038,260, U.S. Pat. No. 4,024,118 and U.S. Pat. No. 4,038,263. At the early stage, impact resistance of products produced with such method was relatively poor due to relatively lower molecular weight (weight-average molecular weight is below 2.0×104). Moreover, humidity resistance, electrical properties and shaping properties are relatively poor due to existence of inorganic salt. To lower fluidity of resin, and satisfy processing requirements, two methods were used to increase molecular weight of resin during early industrialized production of PPS. One method was expected to obtain PPS resin of low cross-linking level by reducing molecular weight for thermal oxidized cross linking; whereas the other method aimed to obtain branch chained PPS resin by adding limited amount of third reaction monomer (normally polyhalogenated aromatic hydrocarbons above trifunctional level). Nevertheless, the resin as obtained through thermal oxidized cross linking was unavailable for spinning; whereas spinning performance of PPS resin as obtained through addition of polyhalogenated aromatic hydrocarbons above trifunctional level was also unsatisfactory.
Phillips Petroleum Company was the only company engaged in production of PPS resin before 1985 due to patent protection. However, other companies began to establish production facilities to produce PPS resin after 1985, and thereby gradually secured their leading positions in production of PPS resin.
TORAY from Japan has performed massive studies on materials and techniques used by sodium sulfide method, and applied numerous Japanese patents (such as TK 2001-261832, TK 2002-265604, TK 2004-99684, TK 2005-54169, TK 2006-182993, TK 2007-9128, TK 2009-57414, TK 2010-53335 and so on), US patent (U.S. Pat. No. 4,286,018), international patent (WO2006-059509) and Chinese patent (CN200480015430.5). Such patents have provided detailed studies on varieties and consumption of polyhalogenated aromatic compounds, sulfides, solvents and polymerization reaction additives. Among them, polyhalogenated aromatic compounds are mainly represented by 1,4-dichlorobenzene and 1,2,4-trichloro-benzene; sulfides are mainly represented by aquo sodium sulfide. N-methyl-2-pyrrolidone (NMP) is used as the solvent; whereas sodium acetate is used as the polymerization reaction additive. Patents as applied by that company have also provided detailed description of technique control; reaction process normally aims at mixed dehydration of organic solvents, sulfides, polyhalogenated aromatic compounds and polymerization reaction additives within the temperature range of 100˜230° C. for the purpose of producing PPS resin through polymerization within the temperature range of 200˜290° C. In order to obtain PPS of higher degree of polymerization, it is essential to proceed with polymerization by numerous stages for extrusion and shaping of PPS resin obtained. However, patents as applied by the company are not involved with isolation or recycling of sodium acetate, the polymerization additive.
KUREHA TECHNO ENG CO., LTD from Japan has also applied numerous Japanese patents for synthesis techniques on PPS resins of different properties (such as TKZ 62-187731, TKZ 62-253626, TKZ 62-285922, TKZ 63-39926, TKP 6-145355, TKP 8-183858, TK 2000-191785, TK 2004-244619 and TK 2004-51732). Its selection of polyhalogenated aromatic compounds, sulfides, solvents and polymerization additives is similar to that of TORAY. Polymerization aims to obtain acceptable PPS resin through two-stage reaction. According to requirements, polymerization additives are to be added at different polymerization stages. Furthermore, mole ratio of H2O/S is normally over 1.0 at polymerization stage. It is also essential to supplement water at the later stage of polymerization to increase mole ratio of H2O/S to 2.5˜3.0. This may significantly increase the reaction pressure, which has put forward higher requirements for reaction devices. Chinese Patent CN88108247.3 as applied by the company in China has also mentioned similar PPS resin synthesis techniques. To improve performance of final products, patents as applied by the company have provided numerous reports on after-treatments such as pickling.
Polymerization techniques as described in Japanese patents applied by Tonen Chemical Corporation (such as TKP 5-222196, TKP 6-157756, TKP 7-102065, TKP 7-224165 and TKP 7-292107) are also in two-stage reaction mode. To obtain PPS resin of higher molecular weight, cooling reflux device is added to the gas phase of the reactor in addition to supplement of polymerization additives and trichloro-benzene. This aims to minimize degradation by-reaction.
Japanese patents (such as TKP 5-78487 and TKP 5-78488) applied by Tosoh Finechem Corporation propose to add multifunctional monomers during polymerization for co-condensation to obtain PPS of higher molecular weight. Japanese patents (such as TKP 3-43692 and TKP 5-140328) as applied by the company also propose to proceed with oxidized cross linking of synthetic PPS resin to reduce melt mass-flow rate (MFR), and improve mechanical performance of PPS resin.
US patents (U.S. Pat. No. 4,490,522, U.S. Pat. No. 4,507,468 and U.S. Pat. No. 5,169,892) applied by Dainippon Ink Chemicals Inc has also provided a description of method for synthesis of PPS resin through single-stage or multi-stage polymerization. As proposed in US patents (U.S. Pat. No. 6,369,191 and U.S. Pat. No. 6,600,009), a coolant is supplied to the cooling device at the top or inside of the reactor at the later stage of heat preservation during polymerization to reduce the pressure inside the reactor. Once the polymerization is completed, proceed with cooling to the specified temperature before adding acetic acid, oxalate, formic acid, chloroacetic acid, hydrochloric acid and sodium bisulfate for acid treatment of slurry. PPS resin as obtained in this way may have a crystallization temperature over 220° C., a whiteness of 50-65 and a maximum viscosity of 240 Poise.
Japan is a leading PPS resin producing country at present. Most of companies in Japan focus on systematic studies of polymerization process and post treatment techniques other than recycling of solvents and additives.
Studies on synthesis and application of PPS resin in China were started in 1970s. Relevant research and manufacturing enterprises have also applied some Chinese patents (CN85102554A, CN85109096A, CN00116141.5, CN00120629.X, CN02113673.4, CN200510022437.6 and so on). Presently, major PPS resin manufacturers in China include Sichuan Deyang Chemical Co., Ltd., Zigong Honghe Chemical Co., Ltd. and so on. Sodium sulfide and p-dichlorobenzene are frequently used as materials for PPS synthesis in China; whereas synthesis techniques are also similar to sodium sulfide based techniques abroad. To improve product performance, a large quantity of additives is added during dehydration, synthesis and post treatment.
Meng Xiao et al. (Modern Chemistry, 32(2), 36-40) propose to proceed with solid-liquid separation of water contained slurry upon completion of polymerization by using lithium chloride as the reaction additive; water contained in the filtrate obtained is to be removed through depressurization and distillation prior to heat preservation, sedimentation and removal of inorganic salt through filter; whereas residual filtrate is to be directly used for follow-up PPS synthesis. Despite of the fact that such method for treatment of filtrate is relatively simple, it is still necessary to increase the proportion of solvent during polymerization in view of limited solubility of lithium chloride in NMP.
Water containing sodium sulfide is normally in solid form, which may produce numerous impurities through oxidization during transport. This is unfavorable for synthesis of PPS resin of high molecular weight. On the contrary, sodium bisulfide is in the form of aqueous solution at a higher concentration, which is unlikely to be oxidized during transport. Therefore, it is more favorable for accurate measurement. For this purpose, various PPS manufacturers have carried out studies on polymerization techniques using sodium bisulfide as the material.
According to the patent (TK 2010-70702) applied by TORAY, sodium bisulfide is mixed with p-dichlorobenzene, sodium hydrate and NMP for direct heating to produce PPS of low molecular weight through polymerization, and thereby obtain oligomer with weight-average molecular weight of approximate 1.2×104 through separation of reaction product; Further mixing such oligomer with limited sodium bisulfide, sodium hydrate, p-dichlorobenzene and NMP for directly heating and polymerization to obtain PPS product with weight-average molecular weight of approximate 2.5×104 through separation. Nevertheless, the quantity of synthetic low polymer as obtained with this method is extremely low. Patents (CN200310123491.0, CN200580039249.2, CN200780017569.7 and CN200780102158.8) as applied by the company in China have also proposed the method for synthesis of PPS resin through dehydration and addition of p-dichlorobenzene for multi-stage polymerization by taking sodium bisulfide as the material, sodium acetate as the additive and NMP as the solvent.
According to patent (TK 2004-244619) applied by KUREHA TECHNO ENG CO., LTD, sodium bisulfide is mixed with NMP for heating and dehydration prior to polymerization through heating with addition of p-dichlorobenzene. Once the temperature is increased to 180° C., further dropping sodium hydroxide solution to control pH value of the reaction system prior to further heating to the temperature of 230° C.; supplement water (mole ratio of H2O/S is normally above 2.0) for eventual heating to the temperature of 260° C. until the reaction is completed; Finally, proceeding with treatment to the reaction product to obtain PPS resin. Patent (CN200380107629.6) as applied by the company in China has also proposed to proceed with normal dehydration and multi-stage polymerization with addition of p-dichlorobenzene by taking sodium bisulfide as the material and NMP as the solvent. Once the polymerization is completed, filter the reaction slurry, and use acetone for washing, leaching and pickling of filter cake prior to further washing and drying to obtain granular PPS resin of different viscosities.
Dehydration process in aforesaid synthesis technique using sodium bisulfide is similar to the technique using sodium sulfide. The only difference is the addition of reaction between sodium bisulfide and sodium hydroxide. Furthermore, sodium bisulfide is relatively poor in stability with loss between 1.0-3.5 mol %. It has brought difficulties to synthesis of PPS of higher molecular weight.
In 1991, Darryl R. Fahey and Carlton E. Ash (Darry R. Fahey, Carlton E. Ash. Mechanism of Poly(p-phenylene sulfide) growth from p-Dichlorobenzene and Sodium Sulfide. Macromolecules, 1991, 24, 4242-4249), carried out thorough study on chemical reactions during heating of water contained sodium sulfide and NMP. Substances as contained in the system are normally indicated as Na2S.NMP.H2O by the empirical formula after dehydration. However, according to analysis and inference by the research personnel through nuclear magnetic resonance, more correct one is the mixture of sodium4-(N-methylamino)-butanoate (SMAB) and sodium bisulfide, namely SMAB-NaHS mixture. SMAB-NaHS mixture is available for better dissolution in NMP of certain temperature. Nevertheless, anhydrous Na2S and NaHS are almost insoluble. It is applicable to obtain such mixture through heating of mixed SMAB and NaHS or NaHS, NaOH and NMP. Furthermore, it is also easy to obtain SMAB through reaction by heating mixed NaOH and NMP.
In conclusion, it is normally required to add polymerization additives for synthesis of PPS resin of high molecular weight that is appropriate for spinning. Common polymerization additives as reported in literatures are mainly represented by sodium acetate and lithium chloride. Sodium acetate and lithium chloride might be partially in solid form upon completion of reaction due to their limited solubility in NMP. This part of sodium acetate or lithium chloride is unlikely to be separated from sodium chloride, the reaction by-product; whereas sodium acetate or lithium chloride dissolved in NMP is to be in the form of salt, which is to be converted into raffinate during distilled recycling of NMP. Therefore, sodium acetate and lithium chloride, the two conventional polymerization additives, are unlikely to be recycled.